The document discusses the past perfect tense and how it is used to indicate that an action was completed before another past action. Specifically:
1) The past perfect expresses that one action occurred before another in the past or before a specific time in the past.
2) It is used to clarify the order of two past events when the timing is important to understand.
3) The construction of the past perfect involves using "had" plus the past participle of the main verb.